The Zarif Ashton meeting is expected to start Monday afternoon in Istanbul, the diplomatic source said.

After Al-Monitor’s report on the unannounced meeting Sunday, Iran’s IRNA news agency reported that Zarif and Ashton were to meet in Istanbul this afternoon and continue the next day.

The meeting comes as negotiators from Iran and six world powers are regrouping after sobering final deal talks in Vienna this month at which both sides seemed taken aback by the others’ positions, and urged their counterparts to return to the talks with more “realism.”

It is unclear why there seemed to be unusual secrecy about the Zarif-Ashton meeting, or why, if it’s to take place, it hasn’t been announced. No American officials are expected to participate in the meeting, Al-Monitor understood.

“I can’t confirm anything. i have no comment,” a spokesperson for Ashton told Al-Monitor early Monday, after Iranian media reported Zarif’s plane had departed for Istanbul to meet Ashton there. Ashton was thought to be returning from South Korea on Sunday, an EU official earlier said.

Zarif met with his Turkish counterpart Foreign Minister Ahmet Davutoglu last week on the sidelines of an Asian summit in Shanghai, China, but no visit was announced. Zarif is expected to attend a conference of the Non Aligned Movement (NAM) in Algeria later in the week.

At the first real comprehensive nuclear deal negotiating round in Vienna earlier this month, the Iranians were said to be shaken by a P5+1 proposal for a ten-year (or by one account, 20-year) plan for phased sanctions relief in a final deal, that Iran found much too slow. The Iranians were also described as having felt the U.S. position in particular had hardened in the most recent Vienna meeting, sources said. (US officials deny the US position has hardened or changed). The US also reportedly raised the ballistic missile issue with the Iranians, which Iran’s negotiators have repeatedly said they refuse to discuss as they consider the missile program a sovereign defense issue outside of the P5+1’s nuclear purview.

Meantime, former US officials close to the US negotiating team have repeatedly implored Iranian negotiators to recognize that Iran’s expectations for the size of its uranium enrichment program have got to be lowered to reach a final deal.

The next round of P5+1 Iran final deal talks is expected to be held in Vienna starting on June 16th. But US, Iranian and EU officials have said there will be additional consultations in different forms and at different levels in advance of those, as the parties aim to try to close a deal by the July 20th expiration of a six month interim deal.

Separately, the US-Iran bilateral channel that helped advance an interim nuclear deal last fall has not yet restarted during the final deal talks this year, outside of bilateral meetings that have taken place on the sidelines of the P5+1/Iran talks, US and Iranian sources told Al-Monitor.